Use of moo3 as corrosion inhibitor, and coating composition containing such an inhibitor

ABSTRACT

The subject of the invention is the use of MoO 3  as a corrosion inhibitor, and an anti-corrosion coating composition for metal parts, characterized in that it comprises:—at least one particulate metal;—an organic solvent;—a thickener;—a silane-based binder, preferably carrying epoxy functional groups;—molybdenum oxide (MoO 3 );—possibly a silicate of sodium, potassium or lithium, and;—water.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of prior U.S. application Ser. No.10/416,375 filed on May 12, 2003, which is a U.S. National Stage ofPCT/IB2001/02764 filed Nov. 12, 2001, which claims priority of FR0014534 filed Nov. 13, 2000.

The object of the present invention is to develop an anti-corrosioncoating for metal parts, preferably a coating free of hexavalentchromium, which is endowed with improved anti-corrosion properties.

The invention applies to metal parts of any type, in particular made ofsteel or cast iron, which need to have good corrosion behaviour, forexample because of their application in the motor-vehicle industry. Thegeometry of the parts to be treated is of little importance as long asthe anti-corrosion compositions may be applied by reliable andindustrializable processes.

One of the objects of the present invention is in particular to improvethe anti-corrosion properties of parts treated without using acomposition based on hexavalent chromium in the formulation of thecoatings.

Many anti-corrosion treatment solutions based on hexavalent chromiumhave been proposed to date. Although they are generally satisfactorywith regard to the protection of treated metal parts, they are, however,becoming increasingly criticized because of their consequences withregard to the toxic risks that they entail and in particular because oftheir adverse consequences for the environment.

As a consequence, various anti-corrosion treatment compositions free ofhexavalent chromium have been recommended. Some of these compositionsare based on a particular metal, such as zinc or aluminium. However,when such compositions are in the form of an aqueous dispersion theirstability is limited. This precludes long preservation and storagetimes.

Within the context of the present invention, the Applicant hasdiscovered that it is possible to improve the anti-corrosion propertiesand the stability of various anti-corrosion coating compositions byincorporating thereinto molybdenum oxide MoO₃ as corrosion inhibitor.

Hitherto, the use of molybdenum oxide MoO₃ as a corrosion inhibitor insystems of aqueous phase has not been known. Certain molybdates, i.e.MoO₄ ²⁻ ions, have already been presented as corrosion inhibitors.However, the Applicant has been able to show that in a certain number ofconventional anti-corrosion compositions the addition of a molybdate,for example zinc molybdate, in no way improves its properties.

The present invention relates more particularly to the use of molybdenumoxide MoO₃ as an agent for enhancing the anti-corrosion properties of acoating composition based on a particulate metal containing zinc or azinc alloy in aqueous phase. This finding has even been extended tocomposition containing hexavalent chromium. This is another object ofthe invention.

Without in any way wishing to be limited to such an interpretation, itseems that in the particular case of an anti-corrosion coatingcomposition based on a particulate metal, the presence of molybdenumoxide MoO₃ makes it possible to improve the control of the sacrificialprotection exerted by the particulate metal in suspension in thecomposition.

According to one particular feature, the particulate metals have alamellar form, the thickness of the flakes being comprised between 0.05μm and 1 μm and having a diameter equivalent (D₅₀) measured by laserdiffraction comprised between 5 μm and 25 μm the subject of theinvention is more particularly the use of molybdenum oxide MoO₃ in acomposition containing zinc in aqueous phase.

According to another feature of the invention, the molybdenum oxide MoO₃is used in an essentially pure orthorhombic crystalline form, having amolybdenum content greater than approximately 60% by mass.

Advantageously, the molybdenum oxide MoO₃ will be used in theanti-corrosion compositions in the form of particles having dimensionsof between 1 and 200 μm.

More specifically, the subject of the present invention isanti-corrosion coating compositions for metal parts, which comprise:

-   -   at least one particulate metal;    -   an organic solvent;    -   a thickener;    -   a silane-based binder, preferably carrying epoxy functional        groups;    -   molybdenum oxide (MoO₃);    -   possibly a silicate of sodium, potassium or lithium, and;    -   water.

The relative proportions of the various constituents in such acomposition may vary widely. However, it has turned out that the contentof molybdenum oxide MoO₃ is preferably between 0.5 and 7% and even morepreferably in the region of 2% by weight of the total composition.

The particulate metal present in the composition may be chosen fromzinc, aluminium, chromium, manganese, nickel, titanium, their alloys andintermetallic compounds, and mixtures thereof. It should be pointed outhere that if the recommended coating composition is preferably free ofCr^(VI), it may nevertheless contain a certain proportion of metallicchromium. In practice, it has turned out that the presence of zinc ishighly desirable.

Advantageously, the particulate metal content is between 10% and 40% byweight of metal with respect to the weight of the composition.

Preferably, the anti-corrosion coating composition according to theinvention contains zinc and/or aluminium, and preferably comprises zinc.

As indicated above, this type of composition is mainly of aqueous natureand therefore preferably contains from 30% to 60% by weight of water.The composition may nevertheless be enriched by the presence of anorganic solvent, preferably a water-soluble organic solvent, which makesit possible to improve the anti-corrosion performance of thecomposition. For this purpose, the composition will contain, forexample, from 1% to 30% by weight with respect to the total composition.However, it seems to be important not to exceed this organic solventcontent of approximately 30%.

In an advantageous embodiment of the invention, the composition willmake use of an organic solvent, for example consisting of a glycolether, in particular diethylene glycol, triethylene glycol anddipropylene glycol.

According to another feature of the present invention, theanti-corrosion composition also contains from 0.005% to 2% by weight ofa thickening agent, in particular of a cellulose derivative, moreparticularly hydroxymethylcellulose, hydroxyethylcellulose,hydroxypropylcellulose, hydroxypropylmethylcellulose, xanthan gum or anassociative thickener of the polyurethane or acrylic type.

The composition of the present invention may also contain mineralrheologic agents of the silica or organophilic clays type.

Such a composition also makes use of a binder, preferably anorganofunctional silane, used in an amount of 3% to 20% by weight. Theorganofunctionality can be represented by vinyl, methacryloxy and amino,but is preferably epoxy functional for enhanced coating performance aswell as composition stability. The silane is advantageously easilydispersible in aqueous medium, and is preferably soluble in such medium.Preferably, the useful silane is an epoxy functional silane such asbeta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,4(trimethoxysilyl)butane-1,2 epoxide orγ-glycidoxypropyl-trimethoxysilane.

Finally, the anti-corrosion coating compositions according to theinvention may also contain, in addition to the aforementioned organicsolvent, up to a maximum amount of approximately 10% by weight of whitespirit so as to improve the ability of the anti-corrosion compositionsto be applied to the metal parts by spraying, dipping or dip-spinning.

Advantageously, the composition may also contain a silicate of sodium,potassium or lithium, preferably in an amount comprised between 0.05 %to 0.5 % by weight.

Naturally, the present invention also relates to anti-corrosion coatingswhich are applied to the metal parts using the aforementionedcompositions, being applied by spraying, spinning or dip-spinningfollowed by a curing operation at a temperature of between 70° C. and350° C. for a cure time of around 30 minutes.

According to an advantageous embodiment, the anti-corrosion coating willresult from an application operation involving, prior to the curingoperation, an operation of drying the coated metal parts, preferably ata temperature of around 70° C. for approximately 20 minutes. Under theseconditions, the thickness of the coating thus applied is between 3 μmand 15 μm and preferably between 5 μm and 10 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2, respectively, are graphs of salt spray resistance, as afunction of bath age, in the examples presented herein below.

In the examples presented herein below for comparative purposes, varioustypes of corrosion inhibitor were tested within the context of thepresent study, which was carried out in order to improve theanti-corrosion properties of various compositions and in particular ofthe reference composition called GEOMET® which has been described inU.S. Pat. No. 5,868,819 herein incorporated by reference.

These were the main commercially available corrosion inhibitors. Theyhave been listed below by broad chemical category, specifying each timethe origin of the product together with its name and its composition.

Modified zinc phosphates:

Supplier: Heubach:

-   -   HEUCOPHOS® ZPA: hydrated zinc aluminium orthophosphate    -   HEUCOPHOS® ZMP: hydrated zinc molybdenum orthophosphate    -   HEUCOPHOS® SAPP: hydrated strontium aluminium polyphosphate        (SrO: 31%; Al₂O₃: 12%; P₂O₅: 44%; MgSiF₆: 0.3%)    -   HEUCOPHOS® SRPP: hydrated strontium aluminium polyphosphate        (SrO: 28%; Al₂O₃: 12%; P₂O5: 42%)    -   HEUCOPHOS® ZCP: hydrated zinc calcium strontium silicate        orthophosphate    -   HEUCOPHOS® ZCPP: hydrated zinc calcium aluminium strontium        silicate orthophosphate (ZnO: 37%; SrO: 5%; Al₂O₃: 3%; P₂O₅:        18%; CaO: 14%; SiO₂: 14%)    -   HEUCOPHOS® CAPP: hydrated calcium aluminium silicate        polyphosphate (Al₂O₃: 7%; P₂O₅: 26%; CaO: 31%; SiO₂: 28%)

Supplier: Devineau:

ACTIROX® 213: zinc iron phosphates (ZnO: 66%; PO₄: 48%; Fe₂O₃: 37%)

Supplier: Lawrence Industries:

-   -   HALOX® SZP 391: zinc calcium strontium phosphosilicate    -   HALOX® CZ 170: zinc orthophosphate

Supplier: Tayca:

-   -   K WHITE® 84: aluminium triphosphate (ZnO: 26.5 to 30.5%; Al₂O₃:        9 to 13%; P₂O₅: 36 to 40%; SiO₂: 11 to 15%)

Molybdates

Supplier: Devineau:

-   -   ACTIROX® 102: zinc molybdates coupled to zinc-phosphate-modified        agents (ZnO: 63%; PO₄: 46%; MoO₃: 1%)    -   ACTIROX® 106: zinc molybdates coupled to zinc-phosphate-modified        agents (ZnO: 67%; PO₄: 46%; MoO₃: 1%)

Supplier: Sherwin Williams:

-   -   MOLY WHITE® MAZP: ZnO, CaCO₃, Zn₃(PO₄)₂, CaMoO₄    -   MOLY WHITE® 212: ZnO, CaCO₃, CaMoO₄    -   Sodium molybdate: Na₂MoO₄

Borates

Supplier: Buckman:

-   -   BUTROL® 23:calcium metaborate    -   BUSAN® 11M2: barium metaborate

Supplier: Lawrence Industries:

-   -   HALOX® CW 2230: calcium borosilicate

Calcium-doped silica

Supplier: Grace:

-   -   SHIELDEX® AC5

Zinc salts

Supplier: Henkel:

-   -   ALCOPHOR® 827: organic zinc salt

Organic inhibitors

Supplier: Ciba-Geigy:

-   -   IRGACOR® 1930: complex of zirconium and        4-methyl-γ-oxobenzenebutanoic acid    -   IRGACOR® 1405: 4-oxo-4-p-tolybutyric acid with 4-ethylmorpholine    -   CGCI® (IRGACOR 287): polymeric amine salts

Supplier: Lawrence Industries:

-   -   HALOX FLASH® X: boric acid, phosphoric acid, triethanolamine        salts, 2-dimethyl-aminoethanol

Zinc passivators

Supplier: Ciba-Geigy:

-   -   IRGAMET® 42:        2,2[[(5-methyl-1H-benzotriazol-1-yl)methyl]imino]bisethanol    -   IRGAMET® BTA M: 1H-benzotriazole.

EXAMPLE 1

The standard reference GEOMET® composition corresponds to: Deionizedwater 38.60% DPG 10.29% Boric acid  0.65% SYMPERONIC ® NP4  1.51%SYMPERONIC ® NP9  1.64% SILQUEST ® A187  8.66% Zinc* 32.12% Aluminium** 5.08% SCHWEGO FOAM ®  0.4% NIPAR ® S10  0.71% AEROSOL ® TR70  0.53%*Lamellar zinc in the form of an approximately 95% paste in whitespirit: zinc 31129/93 of ECKART WERKE;**Lamellar aluminium in the form of an approximately 70% paste in DPG:CHROMAL VIII ® of ECKART WERKE.

To carry out the various comparative experiments on the aforementionedinhibitors, different baths were obtained by adding 1 g of inhibitor to9 ml of water, the dispersion being maintained for 1 hour, then themixture was added to 90 g of the aforementioned standard GEOMET®composition and then stirred for 3 hours.

The first layer of this composition to be tested was applied using a No.38 Conway bar. The drying was carried out at 70° C. for approximately 20minutes and then the curing was carried out at 300° C. for approximately30 minutes.

The second layer was applied using an identical protocol.

The panels thus treated were then tested in a salt spray. The salt sprayresistance results for the various coatings tested are given in thetable below. TABLE 1 Number of hours Nature of the in salt sprayinhibitor Name of the inhibitor without red rust Reference GEOMET 112Modified zinc GEOMET + ZPA 134 phosphates GEOMET + ZMP 122 GEOMET + SAPP66 GEOMET + SRPP 66 GEOMET + ZCP 66 GEOMET + ZCPP 88 GEOMET + CAPP 66GEOMET + ACTIROX 213 66 GEOMET + HALOX 391 66 GEOMET + K WHITE 84 88Molybdates GEOMET + ACTIROX 102 66 GEOMET + ACTIROX 106 88 GEOMET + MW212 88 GEOMET + MW MZAP 88 GEOMET + Na₂MoO₄ 66 Borates GEOMET + BUTROL44 GEOMET + BUSAN 112 GEOMET + HALOX 2230 66 Various GEOMET + SHIELDEX112 GEOMET + ALCOPHOR 827 66 GEOMET + IRGACOR 1930 88 GEOMET + IRGACOR1405 88 GEOMET + CGCI 88 GEOMET + HALOX FLASH X 66 GEOMET + IRGAMET 4244 GEOMET + IRGAMET BTAM 66 Invention GEOMET + MoO₃* 518*MoO₃: POR from CLIMAX Company

In addition, the more particular salt spray resistance results as afunction of the age of the bath, and therefore of its stability at 4° C.and 20° C. respectively, are given in the appended FIGS. 1 and 2.

Both these figures show very clearly that, in each case, on the onehand, the anti-corrosion performance of the composition containingmolybdenum oxide MoO₃ is markedly improved and that, on the other hand,the anti-corrosion performance is maintained better over time whenmolybdenum oxide is added to the composition.

EXAMPLE 2

Two other types of comparative experiments were carried out, one on aGEOMET® composition and the other on a DACROMET® composition based onhexavalent chromium.

The formulations of these compositions are given in the tables below.TABLE 2 GEOMET ® Concentrations Concentrations in % in % Raw materialswithout MoO₃ with MoO₃ Deionized water 38.60 37.83 DPG 10.29 10.08 BoricAcid 0.65 0.64 SYMPERONIC NP4 ® 1.51 1.48 SYMPERONIC NP9 ® 1.64 1.61SILQUEST ® A187 8.66 8.47 Zinc* 32.12 31.48 Aluminium** 5.08 4.98SCHWEGO FOAM ® 0.4 0.21 NIPAR ® S10 0.71 0.70 AEROSOL ® TR70 0.53 0.52MoO₃*** 0 2*Lamellar zinc in the form of an approximately 95% paste in whitespirit: Zinc 31129/93 of ECKART WERKE;**Lamellar aluminium in the form of an approximately 70% paste in DPG:CHROMAL VIII ® of ECKART WERKE.***MoO₃: POR from CLIMAX CompanySYMPERONIC ®: nonionic surfactantsSILQUEST ® A187: γ-glycidoxypropyltrimethoxysilaneSCHWEGO FOAM ®: hydrocarbon-type antifoamNIPAR ® S10: nitropropaneAEROSOL ® TR70: anionic surfactant.

TABLE 3 DACROMET ® Concentrations in % Concentrations in % Raw materialswithout MoO₃ with MoO₃ Deionized water 47.86 44.90 DPG 15.95 15.63 PGMEacetate 1.56 1.53 Chromic acid 3.81 3.73 REMCOPAL ® 334 0.72 0.71REMCOPAL ® 339 0.72 0.71 Zinc* 23.61 23.14 Aluminium** 3.06 3.00 Boricacid 1.30 1.27 ZnO 1.41 1.38 MoO₃*** 0 2*Lamellar zinc in the form of an approximately 95% paste in whitespirit: Zinc 31129/93 of ECKART WERKE;**Lamellar Aluminium in the form of an approximately 70% paste in DPG:CHROMAL VIII ® of ECKART WERKE.***MoO₃: POR from CLIMAX CompanyREMCOPAL ®: nonionic surfactants.

It should be noted that the molybdenum oxide powder was each timeintroduced into the GEOMET® or DACROMET® bath by dusting. The bath washomogenized by stirring using a dispersive blade at 450 revolutions perminute.

The anti-corrosion compositions tested were applied to 10 cm×20 cm coldrolled low carbon steel panels by coating using the Conway bar, followedby predrying at 70° C. during about 20 minutes, and then cured in anoven at 300° C. for 30 minutes.

In the case of application to screws, the compositions were applied bydip-spinning and then cured under the same conditions as for the panels.

The observed salt spray resistance results according to the ISO 9227standard are given schematically in the following table: TABLE 4 Saltspray resistance* Coating Without With 2% PRODUCT SUBSTRATE weight**MoO₃ MoO₃ Aqueous Panels 32 288 >840 GEOMET ® Aqueous Screws 30 144 504GEOMET ® DACROMET ® Screws 24 600 744*Number of hours of exposure to salt spray before red rust appears.**grams per square meter of coated surface, the thickness of thecoatings are comprised between approximately about 6 μm and about 8 μm.

It is therefore apparent that introducing molybdenum oxide MoO₃ intocompositions in aqueous phase, GEOMET® or DACROMET® containingparticulate zinc, improves the salt spray resistance of the saidcompositions very substantially.

Another aspect of the invention consists in adding an alkaline silicateto the composition in an amount comprised between 0.05 % to 0.5 % byweight.

The addition of alkaline silicate, for example sodium silicate,surprisingly enhances the film cohesion in a worthy way.

This is particularly illustrated in the following comparative examplegiven in Table 5.

EXAMPLE 3

In this example, the cohesion is evaluated by applying a transparentadhesive paper on the coating surface and by quick pulling off. Thecohesion is evaluated according to a scale from 0 (complete pulling offof the coating film) to 5 (no pulling off at all of the coating film).TABLE 5 Composition Composition without silicate with silicate(concentrations (concentrations Raw materials in %) in %) Deionizedwater 38.13 37.96 Dipropylene glycol 10.08 10.08 Boric acid 0.64 0.64Symperonic NP4 ® 1.48 1.48 Symperonic NP9 ® 1.61 1.61 Silane A187 ® 8.478.47 Zinc 31129/93 31.48 31.48 Aluminium CHROMAL VIII ® 4.98 4.98Schwegofoam ® 0.21 0.21 NIPAR S10 ® 0.7 0.7 AEROSOL TR70 ® 0.52 0.52MoO₃ 1 1 Silicate of sodium grade 42 0 0.17 Xanthan gum(1) 0.7 0.7⁽¹⁾Thickening agent in order to control the viscosity of the compositionduring application

The composition is applied onto steel panels which have previously beendegreased, with a Conway rod, in order to obtain a weight of a coatinglayer of 30 g/m². The plates are then cured under the same conditions aspreviously described.

They are then submitted to the salt spray test according to ISO 9227 andto the cohesion test. The results are shown in following Table 6. TABLE6 Without With alkaline alkaline silicate silicate Salt spray 694 720(number of hours before appearance of red rust) Cohesion 1/5 5/5

This table shows that even if the resistance to the cohesion is notsignificantly modified, the cohesion on the contrary, is highlyimproved.

1.-61. (canceled)
 62. A coated metal substrate protected with ananti-corrosion coating which coating is established on said substrate bycuring an applied aqueous anti-corrosion coating composition, whereinthe coating composition comprises: at least one particulate metal; anorganic solvent; a thickener; a silane-based binder; molybdenum oxide;and water.
 63. The coated metal substrate of claim 62, wherein saidcoating composition contains from 0.5% to 7% by weight of the molybdenumoxide.
 64. The coated metal substrate of claims 63, wherein said coatingcomposition contains approximately 2% by weight of the molybdenum oxide.65. The coated metal substrate of claim 62, wherein said coatingcomposition contains from 10% to 40% by weight of the at least oneparticulate metal.
 66. The coated metal substrate of claim 62, whereinthe at least one particulate metal is chosen from lamellar zinc and/orlamellar aluminum.
 67. The coated metal substrate of claim 66, whereinsaid at least one particulate metal comprises lamellar zinc.
 68. Thecoated metal substrate of claim 62, wherein the organic solvent is aglycolether.
 69. The coated metal substrate of claim 62, wherein theorganic solvent is diethylene glycol, triethylene glycol, or dipropyleneglycol.
 70. The coated metal substrate of claim 62, wherein said coatingcomposition contains from 0.005% to 2% by weight of the thickeningagent.
 71. The coated metal substrate of claim 70, wherein saidthickening agent is a cellulose derivative.
 72. The coated metalsubstrate of claim 71, wherein said thickening agent ishydroxymethylcellulose, hydroxypropylcellulose, orhydroxypropylmethylcellulose.
 73. The coated metal substrate of claim70, wherein said thickening agent is xanthan gum or an associativethickener of the polyurethane or acrylic type.
 74. The coated metalsubstrate of claim 62, wherein said coating composition contains from 3%to 20% by weight of silane, as the silane based binder.
 75. The coatedmetal substrate of claim 74, wherein the silane comprisesγ-glycidoxypropyltrimethoxysilane.
 76. The coated metal substrate ofclaim 62, wherein the silane based binder carries epoxy functionalgroups.
 77. The coated metal substrate of claim 62, wherein the organicsolvent contains up to approximately 10% by weight of white spirit. 78.The coated metal substrate of claim 62, wherein said coating compositionfurther comprises a silicate of sodium, potassium, or lithium.
 79. Thecoated metal substrate of claim 62, wherein said coating compositioncontains approximately 30% to 60% by weight of water.
 80. The coatedmetal substrate of claim 62, wherein the curing of said coating is doneat a temperature of between 70° and 350° C.
 81. The coated metalsubstrate of claim 62, wherein the curing of said coating lastsapproximately 30 minutes.
 82. The coated metal substrate of claim 62,wherein said coating is established by curing an applied coatingcomposition which is subjected to a drying operation after being appliedand before being subjected to the curing.
 83. The coated metal substrateof claim 82, wherein said coating composition is subjected to the dryingoperation at a temperature of approximately 70° C. for approximately 20minutes.
 84. The coated metal substrate of claim 62, wherein saidcoating is applied with a thickness of between 3 and 15 μm.
 85. Thecoated metal substrate of claim 84, wherein the coating is applied witha thickness of between 5 and 10 μm.